1. Field of the Invention
The present invention relates to an image forming apparatus including a copier, a printer, and a facsimile, in which a nipping device and a fixing device are incorporated. The nipping device and the fixing device are used for the image forming apparatus, the nipping device is preferably implemented in coordination with the fixing device.
2. Background of the Invention
Generally, a fixing device for a color image forming apparatus, which is different from that for a black and white image forming apparatus, is designed so that the time of nipping between a fixing roller and a press roller is from 40 ms to 50 ms, or longer. The purpose of this design is to improve the color characteristic by adjusting the brilliance degree on the surface of the fixed toner as desired. The extended nipping time is also desirable in order to obtain a transparent characteristic when the toner is fixed on an over-head projector (OHP) sheet, by melting a plurality of color toner.
In this case, the width in which a paper sheet is nipped between the fixing roller and the press roller becomes large to ensure the appropriate time of nipping. In a roller type fixing device in which the paper sheet is nipped between the fixing roller and the press roller directly, the diameter of the fixing roller and the press roller are enlarged in order to achieve a greater amount of pressure generated between the two rollers. Thus, in order to ensure the strength of the rollers, the cored bar and the elastic layer of the rollers become thick. Accordingly, the time needed to increase the temperature of the rollers becomes long due to their increased size and decreased heat conducting charateristics.
Japanese Patent Laid-Open No. 2002-258660, describes a belt type fixing device for a color image forming apparatus, which shortens the temperature rising time. A fixing belt is used to lower the heat capacity and increase the nipping width. In the belt type fixing device, the fixing belt is wound around the press roller and the pressing belt is wound around the fixing roller
In Japanese Patent Laid-Open No. 2002-258660, the fixing belt is suspended by plural support rollers, which are arranged across the fixing belt respectively in order to pinch the fixing belt. The press rollers are also arranged so that the line connecting the rotating axis of the press rollers leans from the vertical line of the fixing belt. Thus, when the press rollers pinch the fixing belt, the press rollers drive the fixing belt, and when the press rollers do not pinch the fixing belt, the support rollers drive the fixing belt. In the fixing device, when the press rollers do not pinch the fixing belt, the velocity of the fixing belt is set higher than the peripheral velocity of the press rollers. The purpose of this velocity setting is to avoid a decrease in pressure between the press rollers because of the slack in the fixing belt shortly after the press rollers pinch the fixing belt.
In the belt type fixing device, it is difficult to accord the velocity of the fixing belt with the peripheral velocity of the press rollers. This is because the belt type fixing device has a wide nipping width. It is known that when the velocity of the fixing belt is not calibrated with the peripheral velocity of the press roller, there occurs an error called ‘image difference’. There are plural causes of image difference. One of the causes is thought to be slipping which results from a difference in friction between the fixing belt and recording sheet, and between the recording sheet and the press roller. Another cause is thought to be the difference of velocity which results from bending of the fixing belt where the recording sheet is nipped. Another cause is thought to be the strain of the elastic layer of the support roller and the press roller.